A German law enforcement agency specialising in politically-motivated crimes has opened an investigation into the suspected poisoning of a former KGB intelligence agent and his historian wife who emigrated to Berlin earlier this year.

Marina and Viktor Kalashnikova have become the latest in a series of Russian dissidents to claim that they were poisoned by their former government after tests showed both had more than 50 micrograms of mercury per litre in their blood, far higher than the normal level of three micrograms.

The Kalashnikovas arrived in the German capital in September, having previously stayed in Ukraine, Poland and Estonia.

The couple allege that their clothing, towels and other possessions were contaminated with mercury by Russian agents during their travels through eastern Europe.

Viktor, who is now a commentator and investigative journalist, says he has previously been specifically warned about his writings.

Leah Ferguson reports.